It is known that an indirect field-oriented (or vector-controlled) motor drive provides high performance torque control of an induction motor drive. It is also known in the art of elevator motor controllers to use indirect field-oriented drives to control an elevator induction motor. Such drives are multi-speed variable frequency drives. It is further known that such drives require precise knowledge of the rotor time constant of the motor to establish field orientation.
One technique to accurately determine the rotor time constant is to analyze the motor in an engineering laboratory using expensive test equipment and several engineering man-hours. However, in modernization or retrofit applications, where a new drive replaces an older drive in an existing elevator system, it is not convenient or cost effective to remove the motor or uncouple the motor from the elevator for evaluation of the rotor time constant parameter.
Another technique to determine the rotor time constant involves dispatching a highly skilled engineer to the job site to tune the drive to the motor using special test equipment. However, such a technique is costly and time consuming and, as such, makes modernizing elevator motor drives unattractive for building owners.
Also, various techniques have been described for modeling the rotor time constant of the motor. One technique is described in T. M. Rowan, "A Simple On-Line Adaption for Indirect Field Orientation of an Induction Machine", IEEE Transactions on Industry Applications, Vol. 27, No. 4, July/August 1991; however, such technique does not provide accurate gain adjustment when the direction of rotation of the motor is reversed, such as occurs with elevator motors which are bi-directional. Another technique is described in C. Wang, et al, "An Automated Rotor Time Constant Measurement System for Indirect Field-Oriented Drives", IEEE Transactions on Industry Applications, Vol. 24, No. 1, January/February 1988; however, such technique requires that the torque constant and load inertia are accurately known beforehand.